New spray technology has been developed for spraying solvent-borne compositions with markedly reduced solvent emissions by using environmentally acceptable supercritical compressed fluids such as carbon dioxide as a substitute for the solvents that are normally needed to obtain low spray viscosity. For coating compositions, solvent reductions up to 80 percent have been demonstrated, because only enough solvent for film coalescence and leveling is used.
Although the supercritical fluid spray methods have been highly successful, one difficult problem that is created is that the reformulated composition, which is called a concentrate, has much higher solids level and consequently much higher viscosity after the dilution solvent is eliminated, typically being 800 to 5000 centipoise or higher. Only when the concentrate is mixed with supercritical fluid is a low viscosity obtained. This makes material handling much more difficult than with conventional compositions that contain diluent solvents and therefore have a low solids level and low viscosity, typically below 100 centipoise. Nevertheless, a variety of concentrates have been manufactured at high solids levels and then successfully sprayed with compressed carbon dioxide.
Unexpectantly, however, another difficult problem has been discovered for some solvent-borne concentrates that contain a high concentration of solid polymer, such as some air-dry lacquer coating concentrates that contain high molecular weight polymers, so that they form a hard coating film after the solvents evaporate. Compressed fluids such as carbon dioxide and ethane have sometimes been found to frequently and repeatedly cause severe precipitation of the solid polymer when admixed with the concentrate to prepare a spray solution for spraying. The solid precipitate plugs the mixing apparatus and shuts down the spray operation. Coating concentrates containing cellulosic polymer such as nitrocellulose or cellulose acetate butyrate have been found to be particularly problematic. This has hindered implementation of commercial spray operations using these types of materials.
When this plugging problem was discovered, it was believed that the polymer precipitation was caused by the fluid mechanics of the admixing process, that is, by how the viscous concentrate and inviscid compressed fluid were physically contacted and the two fluids were mixed together. Therefore, it was believed that the precipitation could be reduced, at least to a tolerable level, by suitably controlling the fluid dynamics of the mixing process.
U.S. Pat. No. 5,105,843 discloses an isocentric low turbulence injector apparatus by which carbon dioxide can be injected as a core of fluid completely surrounded by the flow of concentrate such that turbulence at the interface between the two fluids is minimized and desirably maintained as laminar flow as the carbon dioxide dissolves into the concentrate.
Indeed, these mixing apparatus have been found to reduce precipitation and prolong plug-free operation under carefully controlled conditions. However, their performance has generally been disappointing. They have been found to be sensitive to changes or upsets in flow rates and other operating conditions. Often plugging has simply been delayed instead of prevented, so that plugging remains a serious problem for longer term operation. Furthermore, fluid mechanical principles have been unable to successfully predict operating conditions for which plugging does not occur nor have they been able to explain why plugging has occurred at some times and not others. Therefore, operation has had to be by trial and error.
There is therefore clearly a need for an improved method of admixing solvent-borne concentrates with compressed fluids that is reliable, that does not depend upon the particular design of the mixing apparatus, that is not sensitive to flow rate changes, that can substantially reduce or eliminate precipitation instead of just moderating the amount, and that can predict operating conditions under which precipitation and plugging do not occur.